Having problems with this story line too. It’s VERY annoying. Killed undead green thing and can’t do anything else. Guessing the solution is to repeat the quest line until it decides to let me pass the check point when the other guy does one somewhere after the conversation.
The quest is not ticking off investigate the passages below the broken gate, even though I’ve swam every where for about half an hour. Total waste of time and a let down knowing that this bug STILL hasn’t been resolved.